The present invention concerns the field of visual equipment and in particular a device that encourages spectacle wearers to respect a sufficient working distance for close vision. It is been noticed that some spectacle wearers have a disturbing tendency to come excessively close to their reading or working plane in close vision. This invention discloses a device which when associated with single-focus, bifocal or progressive lenses, enables a spectacle wearer to become aware that he is excessively close. The invention discloses a device which incites the spectacle wearer to correct his posture; in the case of progressive lenses, in addition to posture correction, the invention incites the spectacle wearer to utilize the close vision region of the lens for close work. The invention is particularly adapted to myopic children.
At the state of our current knowledge, we know that myopia is generally the result of a complex process. Poor working conditions (bad posture, insufficient lighting . . . ) favor development of myopia. Close vision work occupies a prime place in the everyday activities of school children. In the case of the myopic child, it is his whole organism, and not only vision, which adapts to the ergonomic demands of working conditions.
Myopic spectacle wearers, when they are reading or performing close vision work, have a tendency to come excessively close to the reading or working plane. They do this by adopting a particular position, consisting in bending over the working plane. An article by W. N. Charman (1999), "Near vision, lags of accommodation and myopia", Ophthalmic Physiol Opt, 19: 2, 126-33 showed that moving closer to the reading plane makes it possible to increase the angular size of the object focused on, to increase depth of field by decreasing pupil diameter and reducing the refractive effects of the eye.
The position thus adopted is absolutely uncomfortable, and probably harmful to the back in the case of prolonged and frequent close vision work. Moreover, the fact of bringing the reading plane closer puts greater demands on the visual system. Accommodation and vergence angle increase. A study by M. Rosenfield (1998) "Accommodation and myopia" in: Myopia and near work (Rosenfield M. and Gilmartin G. Eds) ButterworthHeinemann, Oxford 91-116, showed that under these conditions, intra-ocular pressure increases. The increase in accommodation lag, it is supposed, increases retinal fuzziness.
It is consequently essential to encourage myopic children to adopt a suitable position for prolonged work in close vision, not only to prevent back problems, but additionally with an aim to avoiding or stopping development of the child's myopia.
Two types of myopia are conventionally distinguished: congenital myopia and acquired myopia; congenital myopia is present at birth, and generally brings about pronounced myopia; it is been estimated that this represents four to six percent of myopia cases. Acquired myopia appears during the lifetime of the subject. Three groups can be distinguished from subjects suffering from acquired myopia, as a function of the age at which the myopia appears; for the first group, myopia appears towards the age of six years, for the second group at the beginning of adult life between 15 and 20 years, and for the third group at the end of the adult age towards age 50. Grosvenor T. (1987), in "A review and a suggested classification system for myopia on the basis of age-related prevalence and age of onset", Am. J. of Optom. Phvsiol. Opt., 64: 7, 545-54 thus proposes a classification of myopia as a function of the age of onset.
It has also been noticed that there is an evolution of myopia with the passage of time. Grosvenor T. (1980) "Can myopia be controlled? Part 1, Epidemiology of myopia" Optometric Monthly, August, 54-58 established that towards the age of 6/8, five percent of children suffer from myopia of -0.50 diopters or more, whereas the proportion of subjects afflicted by such myopia is 30/35 percent at university entrance age. At the adult age, there appears to be little evolution in myopia, or in any case, a slower change than in children and young adults.
The percentage of myopic persons in a given population appears to depend on academic level, according to Goss D. A. and Jackson T. W. (1995) "Clinical findings before the onset of myopia in youth. Ocular optical components", Optom. Vis. Sci. 72: 12, 870-8), and there appears to be a correlation between the degree of myopia and the number of hours spent each day on close vision work. In Western countries, the proportion of myopia in the population would appear to be between 15 and 20 percent. This proportion reaches 50 percent or even 70 percent in the Asian countries such as China and Japan, according to Yap et al (1994), "Environmental factors and refractive error in Chinese school children", Clinical and Experimental Optometry, 77: 1, 8-14. In African countries, the proportion of myopia sufferers, apart from where this is congenital, is, it appears, practically zero.
It has consequently been advanced that acquired myopia is linked with prolonged close vision work, and that the level of myopia depends on the length of time spent working in close vision. Specialists in optometrics have consequently proposed solutions for attempting to slow down progression of the acquired myopia.
One first solution consists in prescribing a correction that is less than the value of a patient's myopia in close vision; the idea is to reduce the accommodation effort to change from far vision to close vision. See for example Tokoro, T., Kobe S. (1965) "Treatment of myopia and changes in optical components. Report II. Full or under-correction of myopia by glasses", Acta Soc. Ophthamol. Jp., 69, 140-44. This type of method is contested and has not proved it is effective.
Another solution consists in prescribing a correction which is too high for far vision. In fact, ametropy of young hypermetropia sufferers is more stable over time than that of young myopia sufferers, and the idea was to render young myopia sufferers hypermetropic, by prescribing a correction which was too large. See for example, Goss D. A. (1984). "Overcorrection as a means of slowing myopic progression". Am. J. Optom. Physiol. Opt., 61: 2, 85-93. The value of this method has also not been demonstrated.
Authors such as Bates, cited by Grosvenor T. (1980) "Can myopia be controlled? Part 2. The Bates system of eye exercises", Optometric monthly, September. 50-54, have proposed visual exercises: Repeated visual experiences, relaxation, as well as exercising for relieving eye tension. Proof of the effect on the subject's ametropy is not forthcoming.
Finally, specialists in optometrics have attempted to slow down the evolution of myopia by prescribing a power addition for close vision; here, the patient is asked to look through a patch of glass when doing close vision work, this patch having the power needed for close vision. Close vision power addition, it is claimed, has the effect of reducing the close vision accommodation effort, and normalizing phoria in close vision. Investigations are being done into the prescription of bifocal or progressive lenses for children; one can notably consult Goss D. A. Uyesugi E. F. (1995) "Effectiveness of bifocal control of childhood myopia progression as a function of near point phoria and binocular cross-cylinder", J. Optom. Vis. Dev., 26, 12-17 and Mur et al (1991) "Evolution de la myopie d'enfants compensee par progressifs" (Evolution of myopia in children compensated for by progressive lenses) Bulletin de la S.S.C.O., 1, 87-92. The results of studies carried out on the subject show that the prescription of a power addition does not appear effective for all myopias.
There additionally exist various devices employing distance measurement between the frame of the spectacle wearer and the point the wearer is looking at. U.S. Pat. No. 5,359,444 discloses spectacles with the lenses containing a nematic material, of variable refractive index as a function of a voltage applied to the material. The voltage applied to the material is a function of the distance measured by an infra-red telemetry device, whereby the lenses are always focused to the distance of the point observed by the spectacle wearer. The infra-red telemetry device comprises a transmitter and a receiver arranged at each side of the spectacle frame.
U.S. Pat. No. 4,181,408 discloses spectacles with deformable lenses. Deformation of the walls of the lenses changes lens power. The frame thus has a device for measuring angular position of the eyes of the spectacle wearer; an infra-red source illuminates the wearer's eyes, and a sensor detects iris position. Lens power is adjusted as a function of the angular position of the wearer's eyes.
U.S. Pat. No. 5,182,585 discloses spectacles of the same type, having deformable lenses. To adjust lens power, that Patent discloses the use of a telemetry device fastened to the frame rather than measurement of the angular position of the wearer's eyes.
International application WO-A-9717043 discloses spectacles for a blind person. A plurality of infra-red transmitters and receivers is arranged on the surface of sunglasses lenses; related electronics convert distance measurements into measurements able to be perceived by the spectacle wearer.
Progressive multifocal ophthalmic lenses are also known. These lenses have a power which continuously varies, between a nominal power at a reference point for far vision, situated at the top part of the lens, and a reference point for close vision, situated at the bottom part of the lens. These lenses are prescribed for presbytic spectacle wearers; power addition is the difference between the power at the close vision reference point and the power at the far vision reference point. Such lenses are for example disclosed in French Patents 2,769,998, 2,769,999 or yet again 2,770,000.